Hot pressing process



Nov. 26, 1968 3. w. MEADOWS 3,413,392

HOT PRESSING PROCESS Filed Oct. 17, 1966 3 Sheets-Sheet 1 FIG. IA

i INVENTOR 3 GEOFFREY w. MEADOWS BY fi m "6/51;

ATTORNEY Nov. 26, 1968 G. w. MEADOWS 3,413,392

HOT PRESSING PROCESS Filed Oct. 17, 1966 3 Sheets-Sheet 2 20 P" s m l4I). I Y

-21 4 I E m 32 2s n 38 38 J 58 INVENTOR GEOFFREY W. MEADOWS ATTORNEY 5Sheets-Sheet 3 Filed Oct. 17. 1966 INVENTOR GEOFFREY W. MEADOWS ATTORNEYUnited States Patent Oflice 3,413,392 Patented Nov. 26, 1968 3,413,392HOT PRESSING PROCESS Geoffrey W. Meadows, Kennett Square, Pa., assignorto E. I. du Pont de Nemours and Company, Wilmington, Del., a corporationof Delaware Continuation-impart of application Ser. No. 500,609, Oct.22, 1965. This application Oct. 17, 1966, Ser. No. 594,643

5 Claims. (Cl. 264-102) ABSTRACT OF THE DISCLOSURE Oxygen-sensitiverefractory materials are rapidly hotpressed by a repetitive procedure inwhich a refractory material is loaded into reusable shells. The loadedshells positioned in a repetitive sequence are introduced into a housingcontaining an atmosphere essentially free of oxygen, in turn positionedin a preheated susceptor and heated to from 500 to 2500 C. When therefractory material has reached the desired temperature, a pressure offrom 200 to 30,000 pounds per square inch is applied for a sufiicienttime to compact the refractory material to at least 95% of theoreticaldensity. The shell is then immediately withdrawn from the susceptor andthe refractory material is rapidly cooled in an atmosphere essentiallyfree of oxygen, before or after being removed from the shell.

This application is a continuation-in-part application of my copendingapplication Ser. No. 500,609, filed Oct. 22, 1965, now abandoned.

This invention relates to the forming of refractory solids by hotpressing. More particularly, the invention is directed to methods forthe rapid and repetitive hot pressing of refractory materials undercontrolled conditions, in the absence of oxygen, to essentially theirmaximum theoretical density.

Hot pressing has not usually been economically competitive with otherprocesses for fabricating high density metallic and refractory bodies inindustrial quantities. Various other forming methods, e.g. cold pressingand sintering, have been favored since they have required shorterprocess times per unit processed and can be conducted on a continuousbasis. However, hot pressing is a preferred process since it can yieldcompletely dense products where as sintered products often containresidual porosity.

The rapid, bot-pressing process of this invention involved the followingoperations performed in a repetitive sequence:

(a) Loading an oxygen sensitive refractory material in a reuseable shellmade of refractory material and, optionally, preheating the loadedshell,

(b) Introducing the loaded shell into a heated susceptor situated in ahot press, the press preferably having two movable pressing means,

(c) Maintaining the hot press in an atmosphere essentially free fromoxygen,

((1) Heating the loaded shell to a temperature between about 500 and2500 C.,

(e) Applying a pressure between 200 to 10,000 pounds per square inch tothe. refractory material within the shell, the heat and pressure beingprogrammed for a period of time to achieve the desired compaction, i.e.,at least 95 of the theoretical density,

(f) Immediately withdrawing the loaded shell from the susceptor beforethe compacted material has a chance to cool substantially, and in eitherorder,

(g) Cooling the shell and compacted material in an atmosphereessentially free from oxygen, and

(h) Removing the compacted refractory material from the shell, the shellbeing reuseable in the process. While the loaded shell is cooling as setforth in step (g), another shell, having been loaded, can be introducedinto the hot press to compact its contents to the desired density.

In the process, steps (g) and (h) can be performed in either order. Thusthe shell can be cooled with the compacted material remaining inside,the material subsequently being removed from the shell. In thealternative embodiment, the compacted material can be ejected from theshell while hot, and then cooled in the oxygen free atmosphere.

In many embodiments of the process of the invention, depending upon therefractory work material involved and the pressing parameters, it willbe desirable to perform all the steps in the absence of oxygen. Theoxygen free environment can be obtained by locating the equipment withina sealed housing and maintaining a vacuum or an inert gas atmospherewithin the housing.

Optimum temperatures, pressures and times of application can beprogrammed for each material to be formed in the hot press. The heat andpressure can be applied simultaneously or in the particular orderdesired depending upon the refractory material involved and thecharacteristics desired for the compacted product.

The process of the invention through the use of a reuseable refractoryshell containing the refractory material to be compacted provides aneconomical and continuous method of hot pressing. The use of thereuseable shell permits the rapid introduction and removal of manysamples of refractory material into the hot press in sequence, andwithout interruption. This is accomplished since the loaded shell can beinserted into a preheated susceptor and rapidly heated, and moreimportantly, the shell can be removed from the heated susceptor in thehot press without cooling, thus freeing the hot press for another shell.The entire operation within the press for each sample can be carried outin a few minutes rather than hours as was often the case in prior artprocesses.

The process of the invention will further be described with reference tothe following drawings in which:

FIG. 1A is a vertical-sectional view of a loaded shell prior to beinginserted into a hot press,

FIG. 1B is a cross-sectional view along line 1B1B of FIG. 1A,

FIG. 1C is an isometric view of a shell container supporting a pluralityof shells,

FIG. 2 is an elevation with parts in section, of a hot press useful inthe process of the invention; the press containing a shell in positionfor the application of heat and pressure,

FIG. 3 is a horizontal, cross-sectional view of the interior of a hotpress containing a sectioned susceptor,

FIG. 4 is a plan view with parts in section, of a completely enclosedhot press and auxiliary equipment.

FIG. 5 is an elevation with parts in section, of another embodiment ofthe hot press useful in the process of the invention.

The process of the invention can be applied to a variety of refractoryraw materials, i.e. oxides, nitrides, carbides, borides, silicides,beryllides, sulfides and mixtures thereof and these materials bondedwith metals, e.g. tungsten carbide bonded with cobalt. The process isparticularly useful in compacting tungsten carbide, silicon carbide,aluminum oxide, tantalum carbide, titanium carbide and mixtuies thereofwhen these materials are bonded with metals, e.g. cobalt, nickel,nickel-molybdenum alloys, and the like. Prior to loading, the refractoryraw material can be in powdered form, in pre-compacted, or in solidbillet form.

Referring to FIG. 1A, a sample of refractory material 1 to be compactedis loaded into a cylindrical shell 2. With some refractory rawmaterials, i.e. reactive refractory powders, the loading operation willhave to be performed in an atmosphere essentially free from oxygen. Therefractory material 1 is confined within this embodiment and assumes theshape of the shell cavity 3. The shape and dimensions of the shellcavity and the amount of material loaded within it can determine thefinal dimensions of the pressed mass. However, the basic function of theshell is to serve as a container for the material to be hot pressed, andit is not necessarily a molding chamber. The size of the pressed mass isnot critical, but the shapes more frequently used have a diameter of A1to 3 /2 inches and a thickness of /s to 3 inches or more.

The cross section of the shell cavity 3 (see FIG. 1B) can be eitherregular or irregular, or it can be made to form any shape desired forthe finished body. Thus it can be circular, triangular, rectangular,polygonal, oval, with grooves and ribs, etc. It can be tapered from oneend to the other, and if the finished body is to be hollow, it cancontain a core.

The cross section of the shell 2 itself can also be almost any shape;however, it will generally be circular or square; the circular crosssection being preferred since it gives greater strength than the othershapes, and a nondistorting shell is desired. The wall thickness of theshell is not critical and is related to the diameter of the piece beingpressed and the pressure to be applied. The wall thickness must be suchthat it supports the pressure to be applied; thus as the diameter of thepiece being pressed or the applied pressure are increased, the Wallthickness must be increased. The relationship between the variable isnot direct, but can readily be calculated by those skilled in the art.

The shell can be made of any of many refractory construction materialswhich have good strength at high tempcratures such as alumina, zirconia,beryllia, silicon carbide, boron nitride, boron carbide, zirconiumcarbide, molybdenum, tungsten, tungsten carbide, titanium carbide,tantalum carbide, titanium diboride, but it is preferably made ofgraphite. The material selected depends upon the pressures andtemperatures that will be encountered during the pressing operation, andthe size of the parts subjected to the pressure.

Thin end discs or plugs 6 are placed into the shell cavity 3 on top andbottom of the material 1 to be pressed; the bottom disc normally havingbeen placed into the cavity prior to loading the material. These discsare made of a refractory material, such as set forth above, butpreferably graphite, and have a close fit with the walls of the shellcavity. Pistons 7 are then loaded in both ends of the shell, the pistonsalso having a close fit with the cavity walls. When properly loaded, oneend of the piston 7 is in contact with the discs 6 and the other endprotrudes from the shell.

Under some circumstances, e.g. when the loaded shell is transportedwhile in a vertical position, it may be desirable that suitable means bepresent to prevent the pistons 7 from falling out of the shell 2. Thiscan be accomplished by providing the lower part 34 of the piston with asmaller cross section than the part of the piston adjacent the end plugs6. The smaller portion can move in and out of the shell, while a rib 35around the bottom of the shell cavity 3 would limit the downwardmovement of the larger section of the piston and maintain the pistonwithin the shell.

Alternatively, the pistons may be retained within the shell by pin andslot means. Thus a shearable pin 36 could be fitted into a suitable holein the shell and extend through into a slot 37 in the piston. Theslotrunning from the outer end of the piston to a point near the innerend.

In an alternative loading procedure, the refractory material 1 can beplaced in the shell cavity 3 in a series of layers separated bynon-reactive spacers (not shown) of another refractory material. Withthis loading, a number of separate pieces can be pressed in one pressingoperation.

Also in order to increase the capacity, it is possible to use a shellcontainer 5 as illustrated in FIG. 1C. In this embodiment, a pluralityof loaded refractory shells 2 are inserted into closely fitting cavitiesin the container; the container also being made of a suitable refractorymaterial.

The loaded shell can be handled with the material 1 within essentiallyloosely packed, or if desired, 200 to 400 pounds per square inch can beapplied to the pistons 7 to give a more compacted refractory sample forease of handling. At this time, optionally, the shell can be preheatedprior to being taken to the hot press. This step shortens the timerequired in the hot press to bring the refractory material to thepressing temperature.

The shell 2 is then introduced to the hot press, illustrated generallyin FIG. 2.

The body of the hot press 8 is located Within a hermetically sealedhousing 9. The housing is connected by a conduit 10 to a vacuum pump,not shown, the pump maintains the housing at the desired vacuumpressure, usually less than 1 mm. of mercury and preferably less than0.1 mm. The vacuum is necessary because at the normal pressingtemperatures many of the refractory samples and the press partsthemselves would oxidize.

Instead of the vacuum, an inert atmosphere can be maintained within thehousing. Thus an inert gas such as helium or argon can be used, or ifpressing certain nitrides, a nitrogen atmosphere can be used. The inertatmosphere is also helpful in some cases in preventing the dissociationof the refractory compound being pressed, e.g. nitrogen, used withnitrides, but its use can lead to more heat losses through the walls ofthe housing since the inert gas is a better heat conductor than thevacuum.

Two rams or plungers 11 and 12 are located with the hot press such thatthey are coacting in the application of pressure; one ram 11 is locatedat the bottom of the I housing and the other 12 at the top. The rams areused to position the shell 2 within the hot press and also to supply thecompacting pressure. The rams are acted on by suitable means, e.g.hydraulic or pneumatic jacks or presses 46 (see FIG. 4) to produce thedesired pressure. It is preferred that the rams can be movedindependently or simultaneously, and that both rams are movable duringthe application of pressure. This double action gives a more uniformpressure distribution within the shell than does a single action press,i.e. one stationary ram and one movable ram. However, a single actionpress can be used satisfactorily for hot pressing thin samples.

An indicator 13 can be attached to each ram to show the amount of rammovement, thus allowing control of the shells position within the pressand indicating the amount of the compaction of the refractory material 1(illustrated in FIG. 1A and 1B). The ends 4 of the rams which areexposed to the high temperatures of the press are preferably made ofgraphite.

The pressures applied during the process of the invention generallyrange from a minimum of about 200 p.s.i. to 10,000 p.s.i., but in thecase of operating at lower temperatures, e.g. 1000 C. with molybdenum,pressures up to 30,000 p.s.i. could be used, although these aregenerally not necessary. The pressure used at the required temperaturemust be sufficient to compact the material or sample 1 to a density ofat least of theory for the refractory composition involved andpreferably in excess of 99%. In the most preferred case, the sample willbe compressed to a density of 100% of theory.

The body of the hot press 8 is composed of a cylindrical tube 14 havingan induction coil 15 around its outer surface. The coil is connected toa suitable source of electrical power, e.g. a spark gap or highfrequency generator. The thermal insulation tube 14 is usually made of anonconducting material such as silica, quartz, or cement bondedasbestos. The annulus of the thermal insulation tube is filled withthermal insulation 16, i.e. carbon black, carbon fibers or cloth,powdered graphite, etc.

This hot press provides heat by induction, which type of heating permitsrapid heat up, heating to very high temperatures, good temperaturecontrol and rapid and repeated temperature changes. However, dependingon press design and other operating characteristics, resistance heating,dielectric heating, heating by hot vapors or gases, or plasma torchheating can also be used. Resistance heating is often economical in useand instead of an induction coil, carbon resistance rods or cylindricalresistance heating units could be used to supply the heat.

In the embodiment employing induction heating, located within theheating tube is a heated susceptor 17. This means acts to heat theloaded shell 2 while it is located in position for the application ofpressure. The susceptor is designed such that its position is fixedwithin the tube 14 and the shell 2 fits within its interior. Thesusceptor, which is preheated by the induction coil prior to theinsertion of the shell provides a rapid heat up and may also provideextra support to the shell 2, if required, when pressure is applied.

In many embodiments the shell will be positioned within a susceptor andspaced therefrom. In this embodiment, the susceptor does not providelateral support for the shell and the shell is quickly heated solely byradiation. This embodiment is illustrated in FIG. 5 where a shell 2 ispositioned within a susceptor 17 The susceptor in this embodiment isalso the heating element, and as illustrated is a resistance heater inthe form of a cylinder. However, if desired, this embodiment could alsouse induction heating. The heated susceptor can be made of the samerefractory construction materials as the shell, graphite beingpreferred.

The temperature produced within the press, and that obtained very nearthe shell can be measured by an optical pyrometer or a radiantpyrometer. A sight lens 18 in the housing 9 can be aligned with agraphite sight tube 19 extending through the heating tube 14 into theheated susceptor 17. Thus the pyrometer can be used to sense thetemperature of the heated susceptor 17 very near the material in theshell. The pyrometer used (not shown) should be calibrated againstprimary standards and against thermocouples positioned in the shellitself so that the actual temperature of the refractory materials 1 canbe determined from their readings.

Automatic temperature control is possible with the information obtainedfrom the radiant pyr-ometer; the information being used to regulate thepower input into the coil 15. Automatic control is also possible usingthermocouples as the sensing device, but the pyrometers have a muchlonger useful life at temperatures above 1500 C.

The temperatures produced generally range from about 500 C. to 2500 C.,and under most operating condi-- tions a temperature of at least 1000 C.is required to fabricate true refractory materials to high density. Thematerials of construction of the hot press generally impose a maximumtemperature of about 2500 C., since above this temperature most of thematerials used lack sufiicient strength.

To load the hot press, the loaded shell 2 can be placed on a suitableconveyor 21, a roller conveyor as illustrated in FIG. 2, and eitherintroduoed directly into the hot press housing 9 and subsequentlyevacuating the housing, or it can be introduced through a suitable airlock 20 into the hot press housing, the housing being continuallymaintained with an oxygen free atmosphere. The air lock 20 can havesliding valves which are operated by automatically controlled pneumaticcylinders.

In other embodiments, not illustrated, instead of roller conveyors, amechanically rotating round table could be used to transport the shells2 within the housing. For example, a round table, having a plurality ofholes to hold the shells, would be located below the hot press body. Thetable would be rotated by mechanical means and would have positioningmeans to stop rotation once a shell is located over the bottom ram. Or asprocket and chain drive could be used. The chain would have mechanicalmeans to secure and move the shell along a raceway to the pressingposition. The chain would be driven by an electric motor linked to asprocket on one end. The sample can also be propelled along paralleltracks or a trough by means of horizontal pushers or rams.

During movement by the conveying devices set forth above, variousmechanical means can be used to secure the shell, such as a shell holder38 (see FIG. 4). This holder 38. has a cavity in which the loaded shell2 with pistons 7 (see FIG. 1A) is placed; the shell being supported on arecessed ledge 39 in the cavity. In another aspect, the cavity of theshell holder could have a diameter such that the smaller portion 34 ofthe piston (see FIG. 1A) would extend through but the larger portionwould be retained by the recessed ledge 39. If this embodiment is used,there would not be any need in the lower piston for the pin 36 and slot37 arrangement or the rib 35 (see FIG. 1A).

The holder is mounted on and is moved by or on the conveyor used, e.g.belt, rollers, chain, etc. The holder is transported until the shellcontaining cavity is aligned with the positioning rams. The bottom ramwill then move through the cavity, engage the piston 7 and raise theshell and piston assembly into the heated susceptor 17.

Once within the housing, the shell can be preheated in a preheater (notshown in FIG. 2) or placed directly into the hot press. In loading thehot press illustrated in FIG. 2, the roller conveyor 21 moves the shellforward until the initial shell is located between the two movablepressing rams 11 and 12, i.e. until it is located over the bottom ram11. At this time, the bottom ram is manipulated to position the shell inthe thick-walled heated susceptor 17. The top ram 12 can also be used toassist in locating the shell 2 in the desired position within the heatedsusceptor 17. The indicators 13 inform the operator when the desiredplacement has been obtained.

In some applications wherein larger and heavier loaded shells areinvolved, it may be desirable to provide means to position and supportthe shell 2 within the heated susceptor 17 that are independent of therams 11 and 12 and pistons 7. Thus movable hollow tubes or shellpositioners (not shown) made of a suitable refractory material, can belocated such that they are concentric with and surround the rams. Thesetubes would have a diameter such that they only engage the shell, notthe pistons. Thus these tubes will move independent of the rams eitherby spring action or by other means and position the loaded shell withinthe hot press, and once positioned and heated, the rams will move withinthe tubes and apply the compacting pressure. This embodiment would beparticularly useful when shell containers 5 containing a plurality ofloaded shells are used.

An embodiment attaching the hollow tube or shell positioner to the ramby means of springs is shown in FIG. 5. Thus, the tube, 47, supports theshell 2 while the end of the ram 11 supports the piston 7. A similararrangement can be used on the top ram, or, more simply, a separatelymounted tube support, 48, can be used as a stop against which the topedge of shell 2 is held by the force of the springs 49 pushing thebottom tube 47 upward against the bottom edge of the shell 2. With theshell and contents so positioned in the susceptor, pressure can beapplied to the pistons 7 by the rams 11 and 12. Upper ram 12 can movefreely within the fixed cylindrical stop 48. Ram 11 can move the limiteddistance necessary to compact the sample; during the compaction the tube47 around ram 11 remains fixed, but the springs 49 are compressed.

Once in place, due to the fact that the susceptor 17 has been preheated,the shell 2 and refractory material 1 therein are rapidly brought up tothe pressing temperature. Pressure is then applied by the rams 11 and 12to achieve the desired compaction. The optimum temperatures, pressures,sequence of application and times of application can be selected foreach individual material to be pressed.

If a shell container 5 containing a plurality of individual shells isused, as illustrated in FIG. 1C, the container 5 can be handled the sameas the individual shells 2. The rams 11 and 12 would be designed so thatthey would simultaneously apply the same pressure to each of the pistons7.

Upon completion of the desired pressing program, the shell 2 containingthe compacted material is ejected from the heated susceptor 17 throughmanipulation of the rams 11 and 12 and it immediately starts to cool.The shell can easily be removed from the susceptor since the shell doesnot contact the susceptor.

After the ejection of the thin-walled shells, the hot susceptor 17 isready to receive the next loaded shell for another pressing. No time islost heating or cooling between pressings.

If desired, the compacted material can be ejected from the shell 2 whileit is still hot. Whether hot or cold ejection of the material is used isoptional, depending upon the particular material compacted.

Thus for hot ejection, after the hot pressing and removal from the hotpress susceptor 17, the loaded shell would be placed on the shell holder38. The rams would continue their movement such that the pistons 7,plugs 6, and compacted material are ejected from the shell, which issupported by the recessed ledge 39 in holder cavity. The shell assemblyand compacted material would then be cooled in an oxygen freeenvironment.

If the cavity of the holder 38 was such to restrain further movement ofthe larger portion 34 of the pistor as previously described, the holdercould have two interconnected cavities (not shown). One of the cavitieswould have a larger diameter such that it would support the shell 2 butnot the larger portion of the piston. In operation the loaded shellwould be placed in the smaller cavity prior to pressing, and afterwards,the rams would place it on the larger cavity; the conveyor 21 havingmoved the holder during the pressing cycle. The rams would then push thepistons, plugs and compacted material out the larger cavity, while theshell is being supported by the recessed ledge of this cavity.

If hot ejection of the sample from the shell is not used, the shellscontaining the pressed material are placed on the conveyor 21 andtransferred to a cooling zone 23 maintained within the hot presshousing. The cooling zone also has an oxygen free atmosphere. Theinitial shell is cooled within the chamber 23 while the next shell isbeing pressed. After cooling, the shell is moved to the secondautomatically controlled, pneumatic air lock 24 and then it isdischarged from the vacuum atmosphere maintained within the press. Asuitable ram can then be used to remove the compacted material from theshell.

susceptor can be sectioned into two or more parts 33 and these may beattached to double acting horizontal, pneumatic pistons 22 asillustrated in FIG. 3. Pressure can be supplied through the horizontalpistons to support the shell 2 from the pressure applied through thevertical rams 11 and 12 in compressing the sample. This system i Insteadof a fixed, one-piece heated susceptor 17, the

ing heating and pressing, positive support is provided to the shell 2 bythe susceptor sections 33 being closed tightly around the shell 2.

After the pressing has been completed the horizontal pistons 22 can thenbe backed off, opening the sections 33 to permit easy release of theshell 2. Such a procedure results in minimum wear on the parts.

It is also possible to reduce wear on the shell by using a fixed,pressure-fitted liner of a refractory material. As wear develops, it isthen possible to replace only the fixed liner.

On a large scale operation it is advantageous to enclose the wholeoperation by using interconnected chambers which are filled with anoxygen free atmosphere and contain ports with rubber gloves throughwhich operators can carry out manipulations, see FIG. 4. The unloadedshell 2 with the bottom piston 7 and plug 6 in place is introduced intoair lock 31 and placed on shell holder 38. The shell is introduced whilesliding valve 40 is opened and sliding valve 41 is closed. After theshell is inside the air lock, sliding valve 40 is closed and the air inthe chamber is displaced by an inert gas. The shell is loaded withpowdered refractory material from the loading meter 25. The loading isdone by a man standing outside the chamber with his hands in rubbergloves which extend into the chamber. The chamber is then evacuated. Thesliding valve 41 is then opened and the loaded shell 2 on the shellholder 38 is moved to the preheater 27. The preheater, e.g. an inductioncoil, is moved down to enclose the shell by hydraulic cylinder 42.Alternatively, means could be used to raise the loaded shell into afixed heater and subsequently lower it.

After preheating, the shell is moved into the hot press 28 by the rams11 and 12. While in the press the refractory material 1 to be compacted,is subjected to heat and pressure to obtain the desired density, 99% to100% of theoretical. After the hot-pressing cycle is completed, theloaded shell 2 is removed from the heated susceptor 17 by the combinedaction of the two rams and placed on the shell holder 38. The shell 2 isthen moved by the conveyor 21 until it is positioned under the cooler23. The cooler can be a series of copper tubes through which cold Wateris running. The cooler 23 can be lowered and raised from the shellthrough the action of hydraulic cylinder 43. The next shell is thenmoved into position on the ram center line and pressed in the samemanner. No additional susceptor heating or cooling time is requiredbetween cycles.

Upon completion of the cooling, sliding valve 44 is opened and the shellis moved into the previously evacuated exit lock 32. Sliding valve 44 isclosed and the pressure in the exit lock 32 is allowed to increase toatmospheric. Exit lock 45 is then opened and the shell is removed fromthe hot press.

The pressed refractory material 1 is then removed from the shell 2 by asuitable hydraulic or pneumatic ram (not shown) and the shells arereturned to the air lock 31 for reuse. if desired, the ram used toremove this material from the shell can be located within the housing,however, this is not necessary.

The figures and specifications have heretofore disclosed hot pressingwhich can be described as operating in a vertical fashion, i.e. the ramsmoving in a vertical direction. The invention is not so limited, and canbe performed with apparatus operating on a horizontal plane with minorchanges. I

For example, the rams could be movable in a horizontal plane and theshells moved through the housing in a direction parallel to the axis ofthe rams. The shells would be placed on a suitable conveyor such thatthe axis of the pistons would be parallel to the axis of the rams.Suitable positioning means could be used to individually transfer ashell assembly from the conveyor and align it with the rams. The ramswould then transfer the shell assembly horizontally into the heatedsusceptor for the compacting operation. After the pressing cycle isover, the rams would remove the shell from the support and place itwhere the positioning means would transfer it back to the conveyor fortransport to the cooling and unloading zones.

The amount of pressure and temperature applied and the period of time ofthe application is very important in the hot pressing of refractorymaterials. It is known in the art that grain size is a very importantfactor in determining physical properties of refractory compacts; smallgrain size is highly desirable because it leads to high values ofphysical properties, particularly strength and hardness. The use of heatis essential to produce dense, coherent bodies, however, exposure tohigh temperatures for long times during fabrication promotes graingrowth, and samples heated under pressure have higher grain growth thanthose not under pressure at the same temperature. With some refractorypowders having very fine particle size the best results have often beenobtained by raising the temperature vary rapidly, applying the pressurelate in the cycle and holding it at the minimum temperature for minimumtime necessary to get complete density. The process of the invention isof particular advantage when used with these very fine particles sincethe process enables the particles to be held .at the high temperaturefor only a short period of time, thus allowing compaction without unduegrain growth.

For other materials, a short hold period at maximum temperature may bedesirable to permit outgasing, sintering and elimination of the largestpores before final compression. In other cases, it is preferable toapply pressure early in the cycle. In all cases, it is clear thatconditions of temperature, pressure and time are critical, they vary foreach refractory material, and must be regulated precisely, and veryquickly. The subject process permits this control to be accomplished asprecise and rapid as needed, thus resulting in superior products in amuch shorter processing time than heretofore possible.

In the subject processes, fast heat up can be obtained by injecting therefractory material to be compacted and the reuseable shell (which maybe partially preheated) into a hot susceptor to allow the refractorysample to heat rapidly to final temperature through radiation orconduction. The sample is held at temperature only long enough fordesired compaction to take place. Ejection of the loaded shell from thehot zone allows rapid cool down of only the small mass accompanying thesample, and this is done without holding up pressing of the next sample.When combined, these steps allow fast efiicient production offine-grained, hot-pressed bodies that are very strong and hard.

In an exemplary operation of the process of the invention, the followingsteps are performed:

Reference is made to FIG. 1 in connection with the description of theloading of the shell and to FIG. 2 for the hot-pressing operation.

The shell 2 used is cylindrical, having an inside diameter of 1 inch,outside diameter of 1% inches, and a length of 4 inches. The discs 6 are/1 inch in thickness and 1 inch in diameter and the pistons 7 are 1 inchin diameter and 2 inches long. Twenty grams of a powder comprising acommercial a-alumina With an average particle size of 0.3 micron isloaded into the shell 2 as follows:

(1) The bottom separation disc 6 is placed into the loading shell 2.

(2) The bottom piston 7 is put into the loading shell touching thebottom separation disc.

(3) The bottom piston and disc are pushed up into the shell about 1 /2inches.

(4) The twenty grams of the powder is poured into the top end of theshell.

(5) The powder is loaded uniformly into the shell. This is accomplishedby tapping the outside of the shell while it is being rotated.

(6) The top separation disc 6 is placed on top of the loaded powder.

(7) The top piston 7 is placed in contact with the top separation disc.

(8) The piston, separator, and powder assembly is moved until the powderis centered in the shell or both top and bottom pistons extend equallengths out from the shell.

(9) A loading pressure of 200 p.s.i. is applied to consolidate theassembly.

The shell assembly is then placed into a holder and onto the rollerconveyor 21 in the air lock 20 of the hot press. The air lock isevacuated and the shell transported into the hot press housing. Theroller conveyor transports the shell until it has positioned the shellon the center line of the rams 11 and 12, for pick up by the rams.

The graphite susceptor 17 has been preheated by induction to 1000 C. Theloaded shell assembly, in line with the pressure rams, is then heldbetween the pressure rams with a minimum pressure (about 200 p.s.i.) andmoved into the cavity in the hot susceptor. It takes about three minutesof radiation for the shell assembly to reach the temperature of thesusceptor. After this period, 4000 p.s.i. is applied to the powder 1within the shell through the pressure rams and pistons. More heat isapplied by induction to heat the sample to 1500 C. in about fiveminutes. It is held at that temperature for five minutes while the 4000p.s.i. is maintained.

The heating is then stopped, the pressure released and the shellimmediately moved out of the susceptor, again while being held betweenthe pressure rams with a light pressure. It is placed back in the holderand moved away from the center line of the rams. The shell cools down inthe cooling chamber 23 while subsequent shells are being pressed in thesame manner.

When the shell has cooled, it is transported out of the hot presshousing through air lock 24. The compacted material is pushed out of theloading shell with a small hydraulic ram. The powder has been pressedinto a strong, hard, non-porous, completely dense refractory body andthe shell is suitable for reuse.

In another exemplary operation, the following steps are performed.Reference is made to FIG. 1 in connection with the description of theloading of the shell and to FIG. 5 for the hot pressing operation.

The shell 2 used is cylindrical, having an inside diameter of 3 inches,outside diameter of 5 inches, and a length of 8 inches. The discs 6 areinch in thickness and 3 inches in diameter, and the pistons 7 are 3inches in diameter and 4 inches long. Two hundred ninety grams of apowder comprising a commercial a-alumina with an average parti cle sizeof 0.3 micron is loaded into a shell 2 as follows:

(1) The bottom separation disc 6 is placed into the loading shell 2.

(2) The bottom piston 7 is put into the loading shell touching thebottom separation disc.

(3) The bottom piston and disc are pushed up into the shell about 1 /2inches.

(4) The 290 grams of powder is poured into the top end of the shell.

(5) The powder is loaded uniformly into the shell. This is accomplishedby tapping the outside of the shell while it is being rotated.

(6) The top separation disc 6 is placed on top of the loaded powder.

(7) The top piston 7 is placed in contact with the top separation disc.

(8) The piston, separator, and powder assembly is moved until the powderis centered in the shell or both top and bottom pistons extend equallengths out from the shell.

(9) A loading pressure of 200 p.s.i. is applied to consolidate theassembly.

The shell assembly is then placed into a holder and onto a rollerconveyor 21 in the air lock 20 of the hot press. The air lock isevacuated and the shell transported into the hot press housing. Theroller conveyor transports 1 1 the shell until it has positioned theshell on the center line of lower ram 11 for pick-up by the rams.

The susceptor 17 is a resistance heater and has been preheated to 1000C. The loaded shell assembly, in line with the lower pressure ram, isthen raised into the hot susceptor, the shell supported by tube 47 andthe piston 7 supported by the lower pressure ram. The tube and lowerpressure ram continue to raise the shell into the susceptor until theshell contacts and is restrained by the fixed cylindrical support 48. Aminimum pressure (about 200 p.s.i.) is then applied by the upperpressure ram 12 and lower pressure ram 11 to the pistons 7. Spring 49 iscompressed allowing the lower pressure ram 11 to penetrate concentrictube 47. It takes about three minutes for the shell assembly to reachthe temperature of the susceptor. After this period, 4000 p.s.i. isapplied to the powder 1 within the shell through the pressure rams andpistons. More heat is applied by resistance to heat the sample to 1500C. in about five minutes. It is held at that temperature for fiveminutes while the 4000 p.s.i. is maintained. The heating is thenstopped, the pressure released and the shell immediately moved out ofthe susceptor, again while being supported by the tube 47 as the lowerpressure ram withdraws downward through the tube, allowing spring 49 toextend. The shell is then placed back in the holder and moved away fromthe center line of the rams. The shell cools down in the cooling chamber23 while subsequent shells are being pressed in the same manner.

When the shell has cooled, it is transported out of the hot presshousing through air lock 24. The compacted material is pushed out of theloading shell with a small hydraulic ram. The powder has been pressedinto a strong, hard, non-porous, completely dense refractory body, andthe shell is suitable for reuse.

The process of theinvention can be varied to accommodate differentpressing conditions. Thus if it is desirable to reach a temperatureequilibrium prior to the application of any pressure, the followingchanges would be taken with respect to the exemplary operation. First,no initial compacting pressure of 200 p.s.i. would be applied and thetop ram 12 would not be used during the positioning of the shell withinthe hot press support 17. Thus no pressure would be applied to therefractory material prior to reaching temperature equilibrium at themaximum desired temperature. The full pressure could then be appliedrapidly and held for the minimum time to achieve density.

The bodies made by the process of the invention have many uses whereverhard, strong, high temperature resistant materials are required, e.g.cutting tool tips, dies, drills, gage blocks, valve seats, etc.

I claim:

1. A rapid, repetitive, hot pressing process for compacting refractorymaterials comprising:

(a) confining said refractory material in reusable shells,

(b) introducing said shells into a housing containing an atmosphereessentially free from oxygen,

(c) positioning in a repetitive sequence said shells into a preheatedsusceptor,

(d) heating said shells to a temperature between about 500 C. and 2500C. while in said atmosphere essentially free from oxygen,

(e) applying a pressure between 200 to 30,000 pounds per square inch tothe refractory material within said shells until said material iscompacted to at least 95% of theoretical density,

(f) releasing said pressure and immediately withdrawing in sequence saidshells from said susceptor before the compacted material has had achance to cool substantially and in either order,

(g) rapidly cooling said shells and compacted material in an atmosphereessentially free from oxygen and removing said material from saidshells.

2. The process of claim 1 wherein the material compacted is tungstencarbide interdispersed with cobalt.

3. The process of claim 1 wherein the heating is between 500 C. and 2000C. and the pressure is between 200 and 10,000 pounds per square inch.

4. The process of claim 1 wherein the atmosphere essentially free fromoxygen is a vacuum.

5. The process of claim 1 wherein the refractory material is confined ina plurality of separate layers in the shell.

References Cited UNITED STATES PATENTS 2,167,544 7/1939 De Bats -1372,990,602 7/1961 Brandmayr 264-125 X 3,264,388 8/1966 Roach 2641253,294,878 12/1966 Carnall 264 X 3,340,270 9/1967 King 264-332 X ROBERTF. WHITE, Primary Examiner.

R. R. KUCIA, Assistant Examiner.

